Re: Re: Unintialised value?

exit 0 unless exists $pages{$page} and defined $pages{$page} and $pages{$page} == 1;
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Is the exists test really necessary here? Shouldn't the test for defined suffice? In my (admittedly limited :-) experience, a perl variable that is defined necessarily exists, so:

exit 0 unless defined $pages{$page} and
   $pages{$page} == 1;
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should suffice.

But perhaps I'm missing something, and you can show some instance where both tests are needed?

TIA

dave

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Re: Re: Re: Unintialised value? by davido (Cardinal) on Jan 26, 2004 at 03:03 UTC
Is the exists test really necessary here? Well, it depends. There are four things that a hash element can be (in a simple world). True, False, Undefined, or Nonexistant. I consider it at least bad style, and a bad habbit to get into, to use defined to test hash elements that may not even exist. Yes, you will not get a warning for doing so. But let's keep things clear; does a key exist or not, is it defined or not, is its value true or not? Consider the following: `\| * * * V a l u e s * * * Evaluation \| 1 or 'X' \| 0 or '' \| undef \| nonexistant -----------+-----------+---------+-------+------------- boolean \| True \| False \| False \| False defined \| True \| True \| False \| False exists \| True \| True \| True \| False` [download] So the only way to be sure whether you're looking at an undefined element, versus a nonexistant element, is to use exists. If you don't care whether it exists or not, only whether or not it's got a value, use defined, but in the example the OP gave us, he seemed to want to exit if a hash element didn't exist, and exists is the right thing for that job. I find it best to not be ambiguous about things that may bite me later. Dave	[reply] [d/l]

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Re: Re: Re: Unintialised value?
by davido (Cardinal) on Jan 26, 2004 at 03:03 UTC

Is the exists test really necessary here?

Well, it depends. There are four things that a hash element can be (in a simple world). True, False, Undefined, or Nonexistant.

I consider it at least bad style, and a bad habbit to get into, to use defined to test hash elements that may not even exist. Yes, you will not get a warning for doing so. But let's keep things clear; does a key exist or not, is it defined or not, is its value true or not?

Consider the following:

           |        * * * V a l u e s * * *
Evaluation |  1 or 'X' | 0 or '' | undef | nonexistant
-----------+-----------+---------+-------+-------------
boolean    |  True     |  False  | False | False
defined    |  True     |  True   | False | False
exists     |  True     |  True   | True  | False
[download]

So the only way to be sure whether you're looking at an undefined element, versus a nonexistant element, is to use exists. If you don't care whether it exists or not, only whether or not it's got a value, use defined, but in the example the OP gave us, he seemed to want to exit if a hash element didn't exist, and exists is the right thing for that job. I find it best to not be ambiguous about things that may bite me later.

Dave

[reply]
[d/l]